Image encryption has been an area of interest for a number of decades, as it can protect pictorial content, which is dedicated to a targeted community, from being observed by unauthorized or illegitimate viewers. The technology of image encryption has found numerous important applications in, for example, consumer, industrial, commercial, communication, and military sectors. In general terms, the concept of an image encryption can be briefly explained as follows. In an encoder, a source image can be converted into a new form, which generally can be referred to as the ciphertext, with the incorporation of a secret encryption key. The key can be another image or can be a string of symbols. It is preferable that the ciphertext be significantly different from the source image, so that it is meaningless to anyone who observes it directly. In the decoder, the ciphertext can be reverted to the source image if the correct secret key, which can be exclusive owned, possessed, or used by legitimate viewers, is presented.
As the failure of an image encryption method can potentially inflict serious monetary loss and security breach, research on the developing of sophisticated image encryption methods has been conducted vigorously for many years. While developers have been working to come up with an encryption method wherein the secret encryption key cannot be revealed through some form of attacks, at the same time, there are a significant number of people, commonly referred to as the cryptanalysts, who are also attempting to develop methods for deducing the secret key of different image encryption methods through various kinds of attacks.
The emergence of digital holography, fueled by the recent advancement on the computing and the display technologies, has instigated a new direction in image encryption. Being different from a two-dimensional (2-D) optical image (such as one captured by a traditional camera), a hologram can be a 2-D complex image that can be capable of representing a three-dimensional (3-D) image. In general, a hologram image can be comprised of high frequency fringe patterns that can bear little clue on the pictorial content it represents. Due to this significant property relating to holograms, encryption of the hologram of a source image often can be more difficult to attack by cryptanalysts than the encryption of the source image directly. However, conventional holographic encryption techniques can suffer from a number of deficiencies or disadvantages, including, that images (e.g., reconstructed images) that have been reconstructed based on conventional holographic encryption techniques can be undesirably noisy, the cryptographic process can suffer from undesirably poor reconstructed image quality or the data size may have to be decreased in order to try to enhance the relatively poor reconstructed image quality, the decryption process can be undesirably complicated due to the encryption being conducted in both the spatial and spectral domains, the cryptographic process can have undesirably complicated hardware and/or optical setups for the decoder, and/or the cryptographic process still can be vulnerable to attacks, such as correlation attacks and attacks based on the family of plain text attacks.
The above-described description is merely intended to provide a contextual overview relating to digital holograms and cryptography, and is not intended to be exhaustive.